Toll booths on public roadways and highways are notorious bottlenecks in highway travel. The problem has been one of increasing seriousness with the ever increasing volume of highway traffic and a number of systems have been proposed in an attempt to provide rapid identification of vehicles at a toll site without requiring the vehicles to stop or slow down.
Some of these systems employ optical detection while others utilize radio frequency or microwave transponders. These systems have not been successful for a variety of reasons including technical problems associated with the equipment itself, disadvantages caused by the bulkiness and/or expense of the equipment and the like.
A number of systems have been proposed which employ a microwave signal responsive target, such as a card or the like, and these systems possess certain advantages. In such systems, the target comprises passive non-linear elements and a harmonic (usually the second) of the transmitted fundamental is reflected by the target and detected to provide identification. One example of such a system is that disclosed in U.S. Pat. No. 3,631,484 (Augenblick) where a target (harmonic generator) produces, in response to a transmitted signal radiating the target, a signal having frequency which is a combination of a harmonic of the transmitted signal and a frequency shift due to the movement of the target. The doppler shift is significant in the Augenblick system and a frequency measuring device is employed in the system receiver to determine the reflected frequency. In a somewhat similar system disclosed in U.S. Pat. No. 3,798,642 (Augenblick et al) an amplitude modulator is employed in the transmitter and the receiver is tuned to the modulating frequency. The target includes a frequency selective network for producing a coded arrangement of signal components which makes up the reflected signal.
A further system which employs basically similar components is that disclosed in U.S. Pat. No. 3,914,762 (Klensch). In this system a target (identification tag) is employed which produces a beam of energy that is pulse modulated in accordance with a preset identification code. The system receiver receives the reflected beam and generates signals which are representative of the code modulation.
Other patents of possible interest include U.S. Pat. Nos. 3,772,691 (Hansen); 3,798,641 (Preti); 3,918,057 (Van Tol); 3,633,158 (Heibel); 3,377,616 (Auer, Jr.); 3,714,649 (Brouwer, et al); and 3,546,696 (Waters et al) although this listing is not, nor is it representated to be, exhaustive.